Our earlier patent GB 2 274 412 discloses reactors for the treatment of exhaust emissions from internal, combustion engines to reduce the emission of pollutants such as NO.sub.x, carbon monoxide and particulates. The active part of the reactors is a bed of particulate ferroelectric material contained between two gas permeable electrodes across which is applied a potential of the order of tens of kilovolts. In addition to removing particulates by oxidation, especially electric discharge assisted oxidation, there is disclosed the reduction of NO.sub.x gases to nitrogen, by the use of pellets adapted to catalyse the NO.sub.x reduction.
Somewhat similar systems also are disclosed in U.S. Pat. Nos. 3,983,021; 4,954,320 and 5,147,516.
Examples of diesel exhaust particulate filters including gas permeable beds are to be found in European patent application EP 0 010 384; European patents EP 0 244 061; EP 0 112 634; EP 0 132 166 and U.S. Pat. Nos. 4,505,107; 4,485,622; 4,427,418 and 4,276,066.
Problems which occur with those systems which utilise plasmas formed in the exhaust gases as they pass through the bed of particulate material are irregularities in the generation of the plasma due to an uneven distribution of the electric field through the bed of particulate material and arcing or electrical tracking between the electrodes and other parts of the structures of the reactors.
It is an object of the present invention to provide an improved reactor for the treatment of internal combustion engine exhaust emissions.